Insert in your Hiperlink Text Token field, change colors and fonts as requiered. Rounded corner wont work in Outlook. Enjoy
<span style="background: #CC0000; color: white; display: inline-block; width: auto; text-align: center; font-family: Arial, Helvetica, sans-serif; font-size: 14px; border-radius: 10px; text-decoration: none; border: 10px solid #CC0000; display:block-inline; width: 200px;">Save to your Outlook Calendar</span>